With the burgeoning growth of high-definition videos, cloud computing and mobile internet data usage, backbone network architectures supporting faster speed and larger traffic capacity are required to keep place with current needs and future-proof networks. Many 100 G coherent transmission systems have been widely utilized to update the long-haul network, and 400 G coherent transmission systems have been under field test by several service providers all over the world. Metro networks will likely follow the steps of long-haul network at a time delay of two to three years. However, a metro 100 G coherent transmission network is much more sensitive to the footprint, power consumption and cost of the coherent transceiver modules. Silicon photonics based on silicon-on-insulator (SOI) platforms are favored for high speed fiber communication applications due to its characteristics of CMOS process compatibility, low cost, low power consumption and ease of integration. With optical integration on SOI platform, it is possible to reduce cost by minimizing assembly touch points, mechanical adjustments, test apparatuses and the requirement of materials and separated devices.
Fiber coupling efficiency has been an important issue for integrated silicon photonics due to the small overlap between the mode of cleaved fiber and the mode of silicon nano-waveguides. Cantilever structures of silicon dioxide with silicon nitride inverse taper core waveguide have been proposed to achieve high coupling efficiency for transverse electric (TE)- and transverse magnetic (TM)-polarized input optical signal, where mode size converter is realized with the taper core waveguide. Still, TM-polarized optical coupling efficiency remains an unsolved issue when silicon is utilized as the inverse taper waveguide in thin silicon SOI platforms.